The most striking demonstration of the combined (bimodal) nature of
speech understanding appeared by accident. Harry McGurk,
a senior developmental psychologist at the University of Surrey in England,
and his research assistant John MacDonald were studying how infants
perceive speech during different periods of development. For example, they
placed a videotape of a mother talking in one location while the sound of
her voice played in another. For some reason, they asked their recording
technician to create a videotape with the audio syllable "ba" dubbed onto a visual "ga."
When they played the tape, McGurk and McDonald
perceived "da." Confusion reigned until
they realized that "da" resulted from a
quirk in human perception, not an error on the technician's part. After
testing children and adults with the dubbed tape, the psychologists
reported this phenomenon in a 1976 paper humorously titled "Hearing
Lips and Seeing Voices," a landmark in the field of human sensory
integration. This audio-visual illusion has become known as the McGurk effect or McGurk
illusion."
Dominic W. Massaro
& David G. Stork, "Speech
Recognition and Sensory Integration",American Scientist, 1998, vol. 86, p. 236-244.

The McGurk effect has played an important role
in audio-visual speech integration and speech reading. Related links
include the following: